The invention relates to the treatment of Parkinson's disease, sexual dysfunction, and depressive disorders.
Parkinson's disease is a progressive neurodegenerative disorder of the basal ganglia of the brain, which most often becomes apparent after the age of 55. It is a prevalent and prototypic hypokinetic disorder, with akinesia, bradykinesia, rigidity and tremor as the most prominent features. The neurological and psychiatric symptoms, including depression and psychosis, with late dementia, usually worsen with time. The neuropathology of Parkinson's disease reveals a striking and selective loss of the dopaminergic neurons of the nigrostriatal pathway of the brain.
As Parkinson's disease is associated with a loss of the neurotransmitter dopamine, it is commonly treated with drugs which replace or mimic the actions of dopamine. Since dopamine itself cannot pass the blood-brain diffusion barrier, the most commonly used therapy is levodopa (L-DOPA), the immediate precursor of dopamine which is readily decarboxylated by remaining dopaminergic neurons and other amine-producing neurons. A complication of long-term treatment with L-DOPA is the development of rapid fluctuations in clinical state such that the patient changes, often abruptly, between mobility and immobility; this phenomenon is known as the ‘on-off’ effect.
An alternative approach to treatment with L-DOPA is the use of drugs (dopamine agonists and partial-agonists) that mimic the actions of dopamine. Treatment with dopamine receptor agonists has some advantages over treatment with L-DOPA. Unlike L-DOPA, dopamine agonists are effective in patients with advanced stages of Parkinson's disease because their action at postsynaptic receptors is unaffected by the lack of dopamine producing nerve cells that decarboxylate L-DOPA to produce dopamine locally, whereas the denervated dopamine receptors are supersensitive to agonists. Furthermore, there is an increasing interest in the potential of dopamine receptor agonists to provide a neuroprotective effect. Theoretically, such a protective effect might result from (i) a decreased need for the use of L-DOPA, a substance that may cause oxidative stress and perhaps even contribute to further damage of dopamine neurons, (ii) stimulation of dopamine autoreceptors resulting in decreased dopamine synthesis, release, and turnover, resulting in reduced metabolism of dopamine to reactive oxygen species, and (iii) by direct anti-oxidant effects.
R(−)-Apomorphine is a directly acting dopamine agonist at both D1 and D2 receptors, and dopamine autoreceptors, without opiate-like or addictive properties. Apomorphine therapy has led to sustained improvements in Parkinson's disease patients with refractory motor oscillations (on-off phenomena). However, it is difficult to administer owing to its poor bioavailability and extensive first-pass metabolism to inactive metabolites. Therefore, apomorphine is usually administered either by intermittent subcutaneous injection or continuous subcutaneous infusion. Following a single dose, apomorphine has an onset of action of 5-15 minutes, and its effects last for 40-60 minutes.
Direct dopamine agonists, including R(−)-apomorphine, are also effective in the treatment of a number of forms of sexual dysfunction, primarily, but not limited to erectile dysfunction. See Martinez et al., J. Urology 170:2352 (2003).
New compounds that overcome the practical problems associated with apomorphine therapy, particularly its lack of oral bioavailability and short half-life, are needed for the treatment of diseases, such as Parkinson's disease and erectile dysfunction.